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प्रश्न
In a family, the father, the daughter and the son are colour blind, whereas the mother has normal vision. Do you think the son and the daughter have inherited the disease from their father? Work out a cross to justify your answer.
उत्तर
Colour blindness, particularly red-green colour blindness, is typically an X-linked recessive condition. This means the gene responsible for the condition is on the X chromosome.
- Father's genotype: Males have one X chromosome and one Y chromosome. Since the father is colourblind, his X chromosome carries the colourblind gene (Xe). His genotype is XeY.
- Mother's genotype: The mother has normal vision and must have at least one normal X chromosome (Xn). If she were a gene carrier, her genotype would be XnXe; otherwise, it would be XnXe.
Cross 1: Assuming the mother is not a carrier (normal vision, XnXe)
The cross would be:
- Father: XeY
- Mother: XnXn
| Xn (mother) | Xn (mother) | |
| Xe (father) | XnXe (carrier daughter) | XnXe (carrier daughter) |
| Y (father) | XnY (normal son) | XnY (normal son) |
Daughter: Both are XnXe (carriers but not colourblind).
Sons: Both are XnY (normal vision).
Conclusion: If the mother is not a carrier, neither the son nor the daughter would be colour blind.
Cross 1: Assuming the mother is a carrier (normal vision, XnXe)
The cross would be:
- Father: XeY
- Mother: XnXe
| Xn (mother) | Xe (mother) | |
| Xe (father) | XnXe (carrier daughter) | XeXe (colour blind daughter) |
| Y (father) | XnY (normal son) | XeY (colour blind son) |
- Daughters: 50% are XnXe (carriers), and 50% are XeXe (colour blind).
- Sons: 50% are XnY (normal), and 50% are XeY (colour blind).
Conclusion: If the mother is a carrier, there is a 50% chance that the son is colour blind and a 50% chance that the daughter is colour blind.
The son and daughter in this family cannot inherit colour blindness from their father directly because males pass their Y chromosome, not their X chromosome, to their sons. The daughters inherit their father's affected X chromosome but will only be carriers unless the mother is also a carrier.
